The principle purpose of the project is to examine the interaction between mechanical and biological factors in fracture healing. The effect of the type and the stiffness the fixation device on new bone formation in fracture healing will be studied. The major techniques will be morphometric to quantitate and qualitate new bond formation, biochemical testing of the strength of the healed bone, and microsphere arteriolar blockade for blood flow measurements. Initially plates will be compared to external fixators by using each on a standard tibial osteotomy in the same animal. Then the effects of stiffness of immobilization will be examined by using varying configurations of percutaneous pins with the external devices. Six half pins (three above and three below the fracture site) will be compared to four half pins (two above and two below) and biplanar fixation will be compared to one-plane fixation. Finlly we will explore the possible benefits of compression of the external fixation device onfracture healing over fixation in neutral position. Possible bone pressure necrosis due to localized high stresses at both fracture site and at the pin-bone interface will be studied. Pin curvature shown on the x-ray will be used to assess the compressive forces and titanium pins will be used to prevent permanent deformation. Blood flow will be measured at stated intervals using labeled microspheres. At the ned of the experimental period, 120 days, the fracture will be examined for strength of healing by biomechanical testing and the amount and quality of new bone will be estimated by morphometric examination.